Downhole Fluid Control System

ABSTRACT

A fluid flow control device serving as an inflow port from a fluid reservoir (R) to the interior of a production pipe (S) is in the form of a housing ( 3   b,    3   c,    3   d,    3   e,    3   f,    3   g,    3   h,    3   i,    3   j,    3   k,    3   l ). The housing has a primary flow path ( 18 ) and a secondary flow path ( 19 ). The secondary flow path is in fluid communication with a chamber (B) in which is arranged an actuator ( 5 ) for a valve device ( 4 ), the valve device arranged to open and close the primary flow path. At least one flow restrictor ( 1,2 ) is arranged in the secondary flow path, the flow restrictor arranged to provide a pressure to chamber (B) sufficient to actuate the valve to an open position when the fluid flowing through the secondary flow path is oil, and a pressure sufficient to actuate the valve to a closed position when the fluid has a viscosity and/or density less than oil.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation under 35 U.S.C § 120 of applicationSer. No. 15/587,419, filed 5 May 2017 which is a Continuation under 35U.S.C. § 120 of application Ser. No. 14/386,459, filed 19 Sep. 2014, nowU.S. Pat. No. 9,683,429 issued 20 Jun. 2017 which is a National Stageapplication under 35 U.S.C. § 371 of PCT/EP2016/054485 filed 6 Mar.2013, which claimed the benefit of U.S. Provisional applications61/679,805 filed 6 Aug. 2012 and 61/613,515 filed 21 Mar. 2012.

FIELD OF THE INVENTION

The invention concerns the control of fluid flowing into a conduit. Morespecifically, the invention concerns a device and a method ofcontrolling the flow of fluids having different properties. Theinvention is useful in controlling flow of fluids from a subterraneanhydrocarbon reservoir and into a production string. The invented deviceand method are useful for production fluids and in the fluid injectioncontext.

BACKGROUND OF THE INVENTION

A well for producing hydrocarbons from a subterranean reservoir mayextend through the reservoir in a number of orientations. Traditionally,reservoirs were accessed by drilling vertical wells. This is simple andstraight-forward technique, but one which provided limited reservoircontact per well. Therefore, in order to access more of a reservoir perwell, techniques and devices were developed to drill horizontal wells,i.e. turning the well from vertical to horizontal at a predetermineddepth below the surface. So-called multi-lateral wells provide evengreater access to—and contact with—the reservoir.

A major challenge in the production of hydrocarbons from subterraneanreservoirs is to increase the ability to recover the oil that is presentin the reservoir. Today, only a part of the oil in a given reservoir isactually recovered and produced before the field is shut down. There arethus strong incentives for developing new technology in order toincrease production and oil recovery.

Two factors are of particular importance in order to increase productionand rate of recovery from a reservoir:

-   -   obtaining maximum reservoir contact, and    -   preventing negative effects of gas and/or water        penetration/breakthrough (commonly referred to as “coning”).

The reservoir contact is commonly achieved by drilling a number ofhorizontal and/or multi-lateral wells. The negative effects of coningare commonly mitigated by so-called Inflow Control Devices (ICD) placedin the production string wall. Typically, a production string in ahorizontal well comprises a large number of ICDs disposed at regularintervals along its entire length. The ICDs serve as inflow ports forthe oil flowing from the reservoir (normally via the annulus between theproduction string and the well formation) and into the productionstring, and are ports having a fixed flow area. So-called autonomousICDs (AICDs) comprise one or more valve elements and are normally openwhen oil is flowing through the device, but chokes the flow when andwhere water and/or gas enters the flow. The annulus between theproduction string and the casing is typically divided into zones byannulus packers, which is known in the art. One or more ICDs or AICDsare then placed in each zone.

A number of ICDs are known in the art, one being described in U.S. Pat.No. 5,435,393 (Brekke, et al.) disclosing a production pipe having aproduction pipe with a lower drainage pipe. The drainage pipe is dividedinto sections with one or more inflow restrictor devices that controlthe flow of oil or gas from the reservoir into the drainage pipe on thebasis of calculated loss of friction pressure along the drainage pipe,the calculated productivity profile of the reservoir, and the calculatedinflow of gas or water.

The state of the art also includes U.S. Pat. No. 7,857,050 B2 (Zazovsky,et al.) disclosing an apparatus for use in preventing unwanted water orgas and having a flow conduit and a structure defining a tortuous fluidpath proximate the flow conduit, where the tortuous fluid path receivesa flow of fluid. The tortuous fluid path is defined by at least firstand second members of the structure, and the first and second membersare movable with respect to each other to adjust a cross-sectional flowarea of the tortuous fluid path. The cross-sectional area and hence thepressure drop can be adjusted by an external force. However, theexternal control and force is expensive, and the number of sections islimited.

U.S. Pat. No. 7,823,645 B2 (Henriksen, et al.) discloses an inflowcontrol device with a gas or water shut-off feature that can be operatedmechanically or hydraulically from the surface of the well. The devicemay include a bypass feature that allows the inflow control device to beclosed or bypassed via shifting of a sleeve. The flow control device canbe adaptive to changes in wellbore conditions such as chemical make-up,fluid density and temperature. The device may be configured to controlflow in response to changes in gas/oil ratio, water/oil ratio, fluiddensity and/or the operating temperature of the inflow control device.However, the external control and force is expensive and the number ofzones is limited.

Autonomous ICDs (AICDs) represent an improvement of the traditional ICDsmentioned above in that they are self-controlled, i.e. without anyexternal power supply or control.

Examples of autonomous ICDs include US 2008/0041580 A1 (Freyer, et al.)and WO 2008/004875 A1 (Aakre, et al.). While the former describes anautonomous flow restrictor with multiple flow blocking members having adensity less than that of the oil, the latter discloses an autonomousflow-control device having a movable disc which is designed to moverelative to an inlet opening and thereby reduce or increase theflow-through area by exploiting the Bernoulli effect and the stagnationpressure created across the disc.

US 2011/0067878 A1 (Aadnoy) describe a flow controller having a flowrestrictor and a pressure-controlled actuator connected to a valve bodywhich in turn cooperates with a valve opening. On a closing side, theactuator communicates with fluid located upstream of the valve openingand the flow restrictor. On the opening side, the actuator communicateswith a fluid located downstream of the flow restrictor and upstream ofthe valve opening. The actuator is provided with a piston which isseparated from the well fluid by at least one diaphragm-resembling seal,specifically a diaphragm having a spring constant.

US 2008/0041582 A1 (Saetre, et al.) describes a flow control apparatushaving a flow restrictor positioned in the flow path between an exteriorof a tubular and its passage. The flow restrictor has an active chamberand a bypass chamber, and a bypass tubing is disposed within the bypasschamber. The bypass tubing has a constant effective flow area forallowing production fluids to enter the passage from the bypass chamber.Flow blocking members are disposed within the active chamber andcooperate with outlets of the tubular to autonomously vary an effectiveflow area for allowing production fluids to enter the passage from theactive chamber based upon the constituent composition of the productionfluids.

US 2011/0198097 A1 (Moen) discloses a valve assembly for regulatingfluid flow in a horizontal wellbore. A housing is coupled to aproduction tubular, has a chamber which is in fluid communicationthrough a flow channel with an inner annulus formed adjacent to thewellbore. A piston and a biasing member are disposed within the chamber,where the biasing member biases the piston into a first position. A flowpath is defined within the housing and communicable with both theproduction tubular and the inner annulus. The flow path can include oneor more nozzles disposed therein, and the piston can be configured tomove between the first position allowing fluid flow through the flowpath to the production tubular and a second position preventing fluidflow to the production tubular. The position is determined by thepressure drop.

US 2011/0308806 A9 (Dykstra, et al.) describes an apparatus forcontrolling flow of fluid in a tubular positioned in a wellboreextending through a subterranean formation. A flow control system isplaced in fluid communication with a main tubular. The flow controlsystem has a flow ratio control system and a pathway dependentresistance system. The flow ratio control system has a first and secondpassageway, the production fluid flowing into the passageways, where theratio of fluid flow through the passageways relates to thecharacteristic of the fluid flow. The pathway dependent resistancesystem includes a vortex chamber with a first and second inlet and anoutlet, the first inlet of the pathway dependent resistance system beingin fluid communication with the first passageway of the fluid ratiocontrol system and the second inlet being in fluid communication withthe second passageway of the fluid ratio control system. The first inletis positioned to direct fluid into the vortex chamber such that it flowsprimarily tangentially into the vortex chamber, and the second inlet ispositioned to direct fluid such that it flows primarily radially intothe vortex chamber. Undesired fluids in an oil well, such as natural gasor water, are directed, based on their relative characteristic,primarily tangentially into the vortex, thereby restricting fluid flowwhen the undesired fluid is present as a component of the productionfluid.

A common advantage of all the above mentioned examples of AICDs it thatthey contribute to a more even inflow along the well path compared tonozzles in traditional ICDs. The purpose is to delay the gas and/orwater breakthrough as much as possible. However, they all suffer fromthe disadvantage that the production is choked also for the oil. Theresult is an overall increase in the degree of extraction (recovery)around the wells compared with the traditional ICDs, but with asignificant loss of production (barrel/day) during the initial phase ofthe well's lifetime.

Furthermore, solutions such as those disclosed in US 2011/0067878 and US2011/0198097 A1 would neither choke nor close for undesired phases(gas/water) at the moment of their breakthroughs.

US 2008/0041580 A1, WO 2008/004875 A1, US 2008/0041582 A1 and US2011/0308806 A9 all contribute to a ICD character having an autonomicability that to a certain degree choke undesired phases, though not tothe extend of coming to a full, or close to full, halt in the inflow.Publications US 2008/0041580 A1 and US 2008/0041582 A1 would, inaddition, not exhibit any reversible property, that is, the ability toautonomically reopen a valve that has been shut due to entrance ofundesired phases at the moment when oil again starts to flow into thewell.

AICDs having the ability to autonomically close, or almost close, suchundesired phases are also known in the art.

One example is found in the publication U.S. Pat. No. 7,918,275 B2 whichdescribes an apparatus having a flow control member that selectivelyaligns a port with an opening in communication with a flow bore of awell bore tubular. The flow control member may have an open position anda close position wherein the port is aligned with the opening andmisaligned with the opening, respectively. The flow control member movesbetween the open position and closed position in response to a change indrag force applied by a flowing fluid. A biasing element urges the flowcontrol member to the open or the closed position. The apparatus mayinclude a housing receiving the flow control member. The flow controlmember and the housing may define a flow space that generates a Couetteflow that causes the drag force. The flow space may include ahydrophilic and/or water swellable material.

However, a major problem with the solution disclosed in U.S. Pat. No.7,918,275 B2 is that the valve is in closed position at the time ofinstallation, during which the fluid velocity and friction is zero.Hence, there will be no force to actuate the opening. If this problem issolved it would anyway be difficult to control the opening/closing ofthe valve based on the flow friction since the latter is normally smallcompared to the friction of the valve mechanisms. In addition, thefunctionality of any reversible property based of drag force/frictionseems doubtful.

Another example of a document disclosing a solution for an AICD whichmay be autonomically closed is found in publication US 2009/0283275 A1describing an apparatus for controlling a flow of fluid into a wellboretubular. The apparatus includes a main flow path associated with aproduction control device, an occlusion member positioned along the mainflow path that selectively occludes the main flow path, and a reactivemedia disposed along the main flow path that change a pressuredifferential across at least a portion of the main flow path byinteracting with a selected fluid. The reactive media may be a waterswellable material or an oil swellable material.

Hence US 2009/0283275 A1 will for an oil reactive material installed inthe main flow path results in a higher flow resistance during throughputof desired phases such as oil relative to no reactive media. A reactivematerial that stops the water/gas and not the oil is unknown for theinventors. The publication does not make use of a second, pilot flow asthe present invention to overcome any hindering of the main flow.

The publication U.S. Pat. No. 7,819,196 B2 also describe a flowcontroller having a flow restrictor and a pressure-controlled actuatorconnected to a valve body, which in turn cooperates with a valveopening. An osmotic cell is used to operate the actuator, which cell isbeing placed in the fluid flow, whereby the necessary motion of theactuator to drive a valve is achieved by utilising the osmotic pressuredifference between the solution in the cell and the external fluidflow/reservoir in relation to the cell. This concept has been shown towork in accordance with its principles, exhibiting a high initial oilproduction while at the same time closing for undesired phases. However,the solution is dependent on a membrane that should manage the harshwell conditions (high pressure and temperature, fouling, etc.) in asatisfactory way. Such a membrane is presently not known in the field.

The purpose of the present invention is to overcome the shortcomings ofthe prior art and to obtain further advantages.

SUMMARY OF THE INVENTION

The invention is set forth and characterized in the main claims, whilethe dependent claims describe other characteristics of the invention.

It is thus provided a fluid flow control device comprising a housinghaving a fluid inlet and at least one fluid outlet, characterized by afirst fluid flow restrictor serving as an inflow port to a chamber inthe housing, and a second fluid flow restrictor serving as an outflowport from the chamber, and wherein the first fluid flow restrictor andthe second fluid flow restrictor are configured to generate differentfluid flow characteristics; and the chamber comprises actuating meansthat is responsive to fluid pressure changes in the chamber.

In one embodiment, the fluid flow control device comprises a valvedevice arranged between the fluid inlet and the at least one fluidoutlet, and operatively connected to the actuating means.

The first fluid flow restrictor and the second fluid flow restrictor areconfigured to impose its respective different fluid flow characteristicsbased on different fluid properties.

In one embodiment, the first fluid flow restrictor is configured toimpose substantially laminar flow characteristics on a fluid flowingthrough the restrictor, and the second fluid flow restrictor isconfigured to impose substantially turbulent flow characteristics on afluid flowing through the restrictor. In one embodiment, the first fluidflow restrictor is configured to impose flow characteristics based onfluid viscosity, and the second fluid flow restrictor is configured toimpose flow characteristics based on fluid density.

The first fluid flow restrictor may be a porous element and the secondfluid flow restrictor an orifice.

The first fluid flow restrictor serves advantageously as the sole inflowport to the chamber, and the second fluid flow restrictor servesadvantageously as the outflow port from the chamber.

In one embodiment, the housing comprises a primary flow path and asecondary flow path, and the fluid flow restrictors and the chamber arearranged in the secondary flow path. In one embodiment, the valve deviceis arranged to close the primary flow path.

The first fluid flow restrictor may be a part of the valve device and/orthe second fluid flow restrictor may be a part of the valve device.

In one embodiment, the valve device comprises a movable body connectedvia flexible bellows to the housing. In another embodiment, the valvedevice comprises a movable piston arranged for sliding movement insidethe housing.

In one embodiment, the fluid flow control device comprises a fluidrestrictor element configured to progressively choke the flow out of theorifice as the valve device is moved towards a closing position.

It is also provided a method of controlling fluid flow through a housingbased on changes in fluid properties, characterized by:

-   -   allowing at least a portion of the fluid to flow through a first        fluid flow restrictor, into a chamber and out of the chamber via        a second fluid flow restrictor;    -   utilizing the pressure change in the chamber that occurs when a        property of the fluid changes to operate a valve device and        thereby controlling the fluid flow through the housing.

In one embodiment of the method, said property of the fluid comprisesviscosity. In another embodiment of the method, said property of thefluid comprises density. In one embodiment, the method comprisesgenerating a substantially laminar flow by the first fluid flowrestrictor, and generating a substantially turbulent flow by the secondfluid flow restrictor.

The invention utilizes the change in pressure that occurs between twofluid restrictors when the fluid properties (such as viscosity) change.This change in pressure is used to move a body and/or actuate a valve.

Although embodiments of the invention have been described with the flowrestrictors being a porous element and an orifice, the invention isequally applicable to other flow restrictors, such as e.g. a longconduit and/or an abrupt geometry change in a conduit.

The inventive flow control device stops unwanted fluids (e.g. water,gas, steam and CO₂) from entering production flow of a desired fluid(e.g. oil) in a better manner than what known ICDs and AICDs do. Theinvented flow control device is robust and fully autonomous. It isreversible in that the valve device changes position as the properties(e.g. viscosity) of the fluid changes. That is, where for example theflow control device closes when the viscosity decreases (i.e. exposed towater or gas), it opens again when the viscosity increases (i.e. exposedto oil).

There is a significant economical gain in preventing choking of theinitial oil production (present value) and increasing the degree ofproduction due to efficient closure of undesired fluid phases such aswater and/or gas. The estimated increase in the production and recoveryfrom a well, which will be a function of the reservoir and fluidproperties, will be at least 10%. The production cost of the inventivevalve is close to insignificant compared to the potential gain due toincreased oil production.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the invention will be clear from thefollowing description of embodiments, given as non-restrictive examples,with reference to the attached sectional sketches and drawings wherein:

FIG. 1a illustrates a principle behind the invention and the inventedflow control device in a basic form;

FIG. 1b illustrates the correlation between change in pressure insidethe chamber (i.e. between the fluid restrictors), and the change influid viscosity;

FIG. 2 is a principle sketch of the flow control device of theinvention;

FIG. 3 is a principle sketch illustrating a second embodiment of theflow control device according to the invention;

FIG. 4 illustrates a third embodiment of the flow control deviceaccording to the invention;

FIG. 5 illustrates a fourth embodiment of the flow control deviceaccording to the invention;

FIG. 6 illustrates a fifth embodiment of the flow control deviceaccording to the invention;

FIG. 7 illustrates a sixth embodiment of the flow control deviceaccording to the invention;

FIG. 8 illustrates a seventh embodiment of the flow control deviceaccording to the invention;

FIG. 9 illustrates an eight embodiment of the flow control deviceaccording to the invention;

FIG. 10 illustrates a ninth embodiment of the flow control deviceaccording to the invention;

FIG. 11 illustrates a tenth embodiment of the flow control deviceaccording to the invention;

FIG. 12 illustrates a tenth embodiment of the flow control deviceaccording to the invention;

FIGS. 13a and 13b are plots illustrating closing and opening forces foroil and water, respectively, in an embodiment of the invented flowcontrol device configured for autonomously stopping water from enteringa flow of oil; and

FIG. 14 is a plot illustrating closing and opening forces as a functionof pressure in an embodiment of the invented flow control deviceconfigured for autonomously stopping fluid flow at a predeterminedpressure difference.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1a illustrates how a fluid F flows into a conduit 3 a at a firstpressure p₁, through a first flow restrictor 1 and into a chamber Bwhere it attains a second pressure p₂, and then flows through a secondfluid flow restrictor 2 before it exits the conduit 3 a at a thirdpressure p₃. When the fluid flow rate and fluid properties (e.g.viscosity, density) are constant, the pressures (p₁, p₂, p₃) areconstant, and p₁, >p₂, >p₃.

In FIG. 1a , the first flow restrictor 1 is a porous element and thesecond flow restrictor 2 is an orifice.

In general, the flow characteristics through a porous medium may bedescribed using Darcy's law (i.e. laminar flow), expressed as:

$\begin{matrix}{{Q = {\frac{\kappa_{perm} \cdot A}{\mu} \cdot \frac{\Delta \; P}{\Delta \; L}}}{{{where}\text{:}\mspace{14mu} Q} = {{fluid}\mspace{14mu} {flow}\mspace{14mu} {{rate}{\; \;}\left( {{units}\mspace{14mu} {of}\mspace{14mu} {volume}\mspace{14mu} {per}\mspace{14mu} {unit}\mspace{14mu} {time}} \right)}}}{\kappa_{perm} = {{relative}\mspace{14mu} {permeability}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {porous}\mspace{14mu} {medium}\mspace{11mu} \left( {{typical}\mspace{14mu} {unit}\text{:}\mspace{14mu} {Darcy}} \right)}}{A = {{cross}\text{-}{sectional}\mspace{14mu} {area}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {porous}\mspace{14mu} {medium}}}\text{}{\mu = {{viscosity}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {fluid}\mspace{11mu} \left( {{{typical}\mspace{14mu} {unit}\text{:}\mspace{14mu} {centipoise}};{{SI}\mspace{14mu} {unit}\text{:}\mspace{14mu} {Pa}*s}} \right)}}{{{\Delta \; P} = {{differential}\mspace{14mu} {fluid}\mspace{14mu} {pressure}\mspace{14mu} {across}\mspace{14mu} {the}\mspace{14mu} {permeable}\mspace{14mu} {medium}\mspace{11mu} \left( {{typical}\mspace{14mu} {unit}\text{:}\mspace{14mu} {Pa}} \right)}},{and}}{{\Delta \; L} = {{the}\mspace{14mu} {length}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {porous}\mspace{14mu} {medium}\mspace{14mu} {running}\mspace{14mu} {parallel}\mspace{14mu} {to}\mspace{14mu} {the}\mspace{14mu} {fluid}\mspace{14mu} {{flow}.}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

Thus, when flowing through the porous element 1, the fluid will undergoa pressure drop ΔP (from p₁ to p₂ in FIG. 1a ) in accordance withDarcy's law (Equation 1), from which it may be derived that the changein pressure (ΔP) across the porous element is proportional to the fluidviscosity (μ) and the fluid flow rate (Q).

The flow characteristics in a fluid flowing through an orifice oranother restrictor (i.e. turbulent flow), may be expressed as:

$\begin{matrix}{{{\Delta \; P} = {K_{orifice}\frac{\rho \cdot v^{2}}{2}}}{{{where}\text{:}\mspace{14mu} \Delta \; P} = {{differential}\mspace{14mu} {fluid}\mspace{14mu} {pressure}\mspace{14mu} {across}\mspace{14mu} {the}\mspace{14mu} {orifice}\mspace{11mu} \left( {{typical}\mspace{14mu} {unit}\text{:}\mspace{14mu} {Pa}} \right)}}{K_{orifice} = {{orifice}\text{-}{specific}\mspace{14mu} {coefficient}\mspace{11mu} ({dimensionless})}}{\rho = {fluid}\mspace{14mu} {{density}\mspace{11mu}\left( {{unit}\mspace{14mu} {of}\mspace{14mu} {mass}\mspace{14mu} {per}\mspace{14mu} {unit}\mspace{14mu} {of}\mspace{14mu} {volume}} \right)}}{v = {{fluid}\mspace{14mu} {velocity}\mspace{11mu} \left( {{units}\mspace{14mu} {of}\mspace{14mu} {length}\mspace{14mu} {per}\mspace{14mu} {unit}\mspace{14mu} {time}} \right)}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

Thus, when flowing through the orifice 2, the fluid experiences apressure drop (ΔP) (from p₂ to p₃) which may be described by equation 2.The change in fluid pressure across the orifice is almost independent ofviscosity, but proportional to the density and the orifice coefficient,and to the fluid velocity squared.

Therefore, referring to FIG. 1a , the fluid pressure p₂ in the chamberB—between the porous element 1 and the orifice 2—will change if theproperties (viscosity or density) of the fluid changes. This isillustrated graphically in FIG. 1b , showing a first (lower) value forp₂ at a higher fluid viscosity (μ_(high)) and a second (higher) valuefor p₂ at a lower fluid viscosity (μ_(low)). This difference between thevalues for p₂ (ΔP₂) occurring when the viscosity changes (e.g.decreases) may be used to perform work, for example actuate an actuator5, which in turn may move a piston/body and/or a valve (not shown inFIG. 1a ).

Although the invention is explained hereinafter with reference to fluidsflowing through a porous element and an orifice, and utilizing thechange in viscosity, it should be understood that the invention appliesto any combination of fluid flow restrictors where the first providesturbulent flow (completely or substantially) and the other provideslaminar flow (completely or substantially), or vice versa.

In general, the present invention utilizes the change in pressure (ΔP₂)that occurs between two different flow restrictors when subjected tofluids of different properties, e.g. oil and water. These properties maybe viscosity, as described above, but also density, as is evident fromEquation 2. The two flow restrictors are configured to impose differentflow characteristics on the fluids. In the example discussed above, thefirst flow restrictor 1 generates a substantially laminar flow and thesecond flow restrictor 2 generates a substantially turbulent flow.

FIG. 2 is a schematic illustration of one application of the principledescribed above, and illustrates an embodiment of the invented flowcontrol device in a basic form (i.e. seals, gaskets and other requiredancillary parts known in the art are not shown). A fluid flow (F) entersa housing 3 b having a primary flow path (conduit) 18 b and a secondaryflow path (conduit) 19 b. The major portion (F₀) of the fluid (F) flowsthrough the primary conduit 18 b and a valve 4 b (which initially isopen), while a smaller portion (f) of the fluid (F) flows through thesecondary conduit 19 b via a first fluid restrictor 1 in the form of aporous member (generating laminar flow) and via a second fluidrestrictor 2 in the form of an orifice, before it re-enters the primaryconduit 18 b and exits out of the conduit 18 b. When the viscosity (u)of the fluid (F) flow changes, the pressure p₂ in the chamber B situatedin the secondary conduit 19 b (defined by the two fluid restrictors)also changes as described above. For example, if a flow of oil F isreplaced by water, the viscosity decreases and the pressure p₂ increases(as explained above with reference to FIGS. 1a and 1b ).

FIG. 2 furthermore shows (schematically) that an actuator 5 b isarranged in the chamber B. The actuator 5 b is connected viatransmission means 6 (e.g. hydraulic linkage, mechanical linkage orsignal cable) to the valve 4 b. When the fluid viscosity (μ) changes asdescribed above, the difference in values for p₂ (ΔP₂, see FIG. 1b )imparts an actuating force on the actuator 5 b, which in turn operates(e.g. closes) the valve 4 b. Thus, the conduits and fluid restrictorsmay be configured and dimensioned such that (when breakthrough is to beprevented) the valve 4 b automatically closes when the viscosity (μ) ofthe fluid (F) drops below a predetermined level. Thus, in an oilfieldapplication, this device prevents unwanted water and/or gas inflow intoa production string.

Another embodiment of the invented flow control device is schematicallyillustrated in FIG. 3. A housing 3 c is arranged in a flow path betweena fluid reservoir R and the interior of a production pipe S. The housing3 c comprises an inlet 7 in fluid communication with the reservoir R andan outlet 8 in fluid communication with the production pipe S. Insidethe housing 3 c is a valve member 4 c in the form of a movable body orpiston (hereinafter also generally referred to as a body). The body 4 cis supported in the housing 3 c by bellows 9 c comprising a structuraland resilient member such as a helical spring (not shown). The body 4 bfurther comprises a first fluid restrictor 1 in the form of a porousmember. The body 4 c and bellows 9 c define a chamber B inside thehousing 3 c, while a second fluid restrictor 2 in the form of an orificeprovides a fluid outlet from the chamber B.

In use, a fluid flow F (e.g. oil from a subterranean reservoir) entersthe housing 3 c through the inlet 7. Inside the housing 3 c, the majorportion F₀ of the fluid F follows a primary conduit 18 c before it exitsthe housing 3 c through the outlet 8 and flows into the production pipeS. The remaining portion f of the fluid F flows through the porousmember 1 in the body 4 b and into a secondary conduit 19 c defined bythe chamber B before it exits the chamber B through the orifice 2, andflows into the production pipe S. If water and/or gas enters the flow F,causing the overall viscosity μ to drop, the resulting difference invalues for p₂ (ΔP₂, see FIG. 1b ) is serving to exert a pressure againsta body surface 5 c. This change in pressure, acting on the body surface5 c, generates a motive force which serves to close the body 4 c againstthe inlet 7, thus preventing fluid from entering the housing 3 c.

FIG. 4 illustrates yet another embodiment of the invented flow controldevice. The housing 3 d comprises an upper part 11 d and a lower part 12d. The two parts 11 d,12 d are joined together by a threaded connection20 with seals (e.g. o-rings) 16 b. The housing 3 d has an inlet 7 andradially arranged outlets 8. A member 4 d is arranged for movement (inthe figure: up and down) inside the housing 3 d. O-rings 16 a sealbetween the movable member and the housing interior wall. A chamber B isthus defined by the movable member 4 d and the lower part 12 d of thehousing 3 d. The movable member 4 d (in this embodiment: a piston)comprises a first fluid restrictor 1 in the form of a porous member anda second fluid restrictor 2 in the form of an orifice.

This embodiment of the flow control device further comprises a fluidrestrictor element 32, here in the form of a face which serves toprogressively choke the flow out of the orifice 2 as the movable piston4 d is moved towards the sealing surface 14.

In use, a fluid flow F (e.g. oil from a subterranean reservoir) entersthe housing 3 d through the inlet 7. Inside the housing 3 d, the majorportion F₀ of the fluid F follows a primary conduit 18 d before it exitsthe housing 3 d through the outlets 8. A portion f of the fluid F flowsthrough the porous member 1 in the piston 4 d and into the chamber Bbefore it exits the chamber though the orifice 2, and is mixed with theflow from the primary conduit. In this embodiment of the control device,the difference in values for p₂ (ΔP₂) as the fluid viscosity μ changes,is serving to exert a pressure against a piston surface 5 d. This changein pressure, acting on the piston surface 5 d, generates a motive forcewhich serves to close the piston 4 d against the inlet 7. The sealingsurfaces 14 and 15 are brought together, thus substantially preventingfluid from entering the housing 3 d.

FIG. 5 illustrates a further embodiment of the inventive flow controldevice. The housing 3 e comprises an upper part 11 e and a lower part 12e, in which the upper and lower parts 11 e,12 e are joined together by athreaded connection 20 with seals (e.g. o-rings) 16 b. The housing 3 ehas an inlet 7 and radially arranged outlets 8. A member 4 e is arrangedfor movement (in the figure: up and down) inside the housing 3 e, guidedby a supporting structure 17. Resilient bellows 9 e extend between themovable member 4 e and the lower housing 12 e, thus forming a chamber Btogether with the movable member 4 e and the lower part 12 e of thehousing 3 e. The movable member 4 e comprises a first fluid restrictor 1in the form of a porous member, and the lower housing 12 e comprises asecond fluid restrictor 2 in the form of an orifice.

In use, a fluid flow F (e.g. oil from a subterranean reservoir) entersthe housing 3 e through the inlet 7. Inside the housing 3 e, the majorportion F₀ of the fluid F follows a primary conduit 18 e before it exitsthe housing 3 e through the outlets 8. A portion f of the fluid F flowsthrough the porous member 1 in the movable member 4 e and into thechamber B before it exits the chamber B though the orifice 2. In thisembodiment of the control device, the difference in values for p₂ (ΔP₂,see FIG. 1b ) as the fluid viscosity μ changes is serving to exert apressure against a surface 5 e on the movable member and thereby toclose the movable member 4 e against the inlet 7. The sealing surfaces14, 15 are hence brought together, resulting in a substantiallyprevention of fluid F from entering the housing 3 e.

FIG. 6 illustrates a further embodiment of the invented flow controldevice. The housing 3 f comprises an upper part 11 f and a lower part 12f, the upper and lower parts 11 f,12 f being joined together to form aprimary conduit 18 f that runs along the interior walls of the housing 3f from the inlet 7 to radially arranged outlets 8. The joining of thetwo parts 11 f,12 f may for example be obtained by screw connection orwelding (not shown). A piston-shaped member 4 f is arranged fortranslational movement (in the figure: up and down) inside the housing 3f, guided by a suitable supporting structure, thus forming a chamber Bsituated between a lower surface 5 f of the member 4 f and the innerwalls of the lower part 12 f. The movable member 4 f comprises a firstfluid restrictor 1 in the form of a porous member and a second fluidrestrictor 2 b in the form of an orifice, thereby forming a secondconduit 19 f defined by the chamber B. Both the first 1 and the secondrestrictor 2 b extend axially through the member 4 f. The size of theorifice 2 b opening may advantageously be of variable radial width.Likewise, the lower housing 12 f may comprise another second fluidrestrictor 2 c in the form of an orifice. In yet another embodiment anappropriate filter 22 may be arranged at one or more of the outlets 8 toprevent any impurities such as particles to enter (and thus block orlimit) the flow. The movable member 4 f and the lower housing 12 f areconfigured to form a fluid restrictor element or area 32 a, here in theform of a corner opening, which serves to progressively choke the flowout of the orifice 2 b,2 c as the pressure builds up in chamber B and inthe fluid restrictor area 32 a. The purpose of the illustratedprotrusions 23 is to avoid complete closure of orifice(s) 2 during flowof fluid phases having lower viscosities than the desired phases such asoil.

In use, a fluid flow F (e.g. oil from a subterranean reservoir) entersthe housing 3 f through the inlet 7. Inside the housing 3 f, the majorportion F₀ of the fluid F follows a primary conduit 18 f before it exitsthe housing 3 f through the outlets 8. A minor portion f of the fluid Fflows through the porous member 1 in the movable member 4 f and into thechamber B before it exits the chamber B though the orifice 2 b locatedin the movable member 4 f and/or the orifice 2 c located in the lowerpart 12 f. Also in the embodiment of the control device shown in FIG. 6,the difference in values for p₂ (ΔP₂, see FIG. 1b ) as the fluidviscosity μ changes is serving to exert a pressure against the lowersurface 5 f on the movable member 4 f and to close the movable member 4f against the inlet 7. Sealing surfaces 14, 15 on the inside walls ofthe upper part 11 f and the upper surface of the movable member 4 f,respectively, are thus brought together to substantially prevent fluid Ffrom entering the housing 3 f. Due to its reinforcing stagnation effectthe fluid restrictor area 32 a contributes to a more efficient closingof the primary conduit 18 f during entrance of fluid phases having lowviscosities.

FIG. 7 illustrates a further embodiment of the inventive flow controldevice. The housing 3 g constitutes an integral part where its interiorare constructed to form a primary conduit 18 g running along theinterior walls of the housing 3 g from the inlet 7 to one or moreradially arranged outlets 8. A translationally movable member 4 garranged inside the housing 3 g is composed of an upper part 4 gu and alower part 4 g 1, e.g. joined together by a threaded connection (notshown) and seals (e.g. o-rings) 16 g. The upper 4 gu and lower 4 glparts of the member 4 g may be guided by an appropriate supportingstructure (not shown) and configured for opposite directed relativemovements (in the figure: up and down) inside the housing 3 g. A chamberB is thus defined by the interior walls of the assembled member 4 g. Themember 4 g (in this embodiment: a piston) further comprises a firstfluid restrictor 1 in the form of a porous member and two second fluidrestrictors 2 b,2 c, e.g. in the form of an variable and a fixedorifice, respectively, thereby forming a second conduit 19 g defined bythe chamber B. Alternatively the control device may have just oneorifice 2 of either variable type 2 b or fixed type 2 c, or two fluidrestrictors 2 of the same type. As for the embodiment shown in FIG. 6 afilter 22 may be arranged in one or more of the outlets 8 to prevent anyimpurities such as particles to enter and thus block or limit the flow.The purpose of the illustrated protrusions 23 is to avoid completeclosure of orifice 2 c during flow of fluid phases with lowerviscosities than desired phases such as oil.

In use, a fluid flow F (e.g. oil from a subterranean reservoir) entersthe housing 3 g through the inlet 7. Inside the housing 3 g, the majorportion F₀ of the fluid F follows a primary conduit 18 g before it exitsthe housing 3 g through the outlet(s) 8. A portion f of the fluid Fflows through the porous member 1 arranged in the movable member 4 g andinto the chamber B before it exits the chamber B through the orifice 2 blocated on the upper part 4 gu of movable member 4 g and/or the orifice2 c located on the lower part 4 g 1 of the movable member 4 g. Also inthis embodiment of the control device, the difference in values for p₂(ΔP₂, see FIG. 1b ), as the fluid viscosity μ changes, is serving toexert a pressure against surfaces 5 g on the interior walls of themovable member 4 g and therefore to close the upper part 4 gu againstthe inlet 7. The sealing surfaces 14,15 are thus brought together, thussubstantially preventing fluid F from entering the housing 3 g.

FIG. 8 illustrates a further embodiment of the inventive flow controldevice. The housing 3 h constitutes one part, where its interior isconstructed to form a primary conduit 18 h running along the interiorwalls of the housing 3 h from a tangential inlet 7 to an outlet 8. Amember 4 h, in this example formed as a piston, is arranged viaappropriate seals 16 h to the inside of housing 3 h, thereby forming achamber B between an upper surface 5 h of the member 4 h and the upperinterior walls in the housing 3 h. The member 4 h may be movable (inthis embodiment: a piston going up and down) or may comprise bellows (orany other stretchable means) extending at least partly over the radialcross section set by the interior walls of the housing 3 h.Alternatively the member 4 h may be a combination of bellows/stretchablemeans and more rigid material(s). The member 4 h may furthermoreoptionally comprise one or more second fluid restrictors 2 in the formof orifice(s) located e.g. in the center of the member 4 h. Further, oneor more conduits 24 extending within the housing 3 h from the outlet 8to the chamber B have optionally porous element(s) 1 arranged in theconduit(s) 24. The induced whirls at the outlet 8 creates a highpressure area which results in a higher pressure in chamber B, and thusa more efficient closure. The combination of one or more second fluidrestrictors 2 and said conduit(s) 24 constitutes a secondary conduit 19h for flow of a minor portion f of the fluid F.

In use, a fluid flow F (e.g. oil from a subterranean reservoir) entersthe housing 3 h through the tangential inlet 7. Inside the housing 3 hthe fluid F follows a primary conduit 18 h before it exits through theoutlet 8, inducing a high pressure area with whirls. A minor portion fof the fluid F may flow into the conduit(s) 24, optionally through anyporous member(s) 1, further into the chamber B and out though theorifice(s) 2 in member 4 h. Also in this embodiment of the invention,the difference in values for P₂ (ΔP₂, see FIG. 1b ), as the fluidviscosity μ changes, is serving to exert a pressure against the uppersurface 5 h on the member 4 h. The sealing surfaces 14, 15 are broughttogether and thus substantially prevent fluid F from entering thehousing 3 h. Alternatively, if there are no second fluid restrictors 2in the member 4 h the stagnation pressure created in a stagnation area33 and in the chamber B would still effectively force the member 4 hdown and thus substantially prevent fluid F from entering the housing 3h, either by rigid movement or by expansion of the bellows downward, ora combination thereof.

FIG. 9 illustrates another embodiment of an inflow control device,wherein the housing 3 i forms a chamber B having an inlet 7 and anoutlet 8 constituting a first fluid restrictor 1 in the form of anorifice and a second fluid restrictor 2 in the form of an opening withan inserted porous material, respectively, thereby creating a secondconduit 19 i defined by the chamber B. Except for the introduction of aporous material at the outlet 8 creating a mainly laminar flow at itsdownstream side during use, and the construction of the orifice at theinlet 7 creating a mainly turbulent flow at its downstream side duringuse, the structural construction of the device is similar or identicalto the device disclosed in the publication US 2011/0067878 A1, whichhereby is included by reference.

In use, a fluid flow F enters the housing 3 i through the inlet/orifice7,1. If the viscosity of the flowing fluid is sufficiently high, such asoil, a translationally moving member/actuator 4 i comprising a piston 24and spring 25 attached by appropriate seals 16 i inside a second chamber26, is in an open position, i.e. a valve member 27 enabling blocking ofthe outlet 8 has been lifted by the actuator 4 i. This is a consequenceof the corresponding high pressure (p₂) formed inside the chamber B dueto the high resistance set up by the second fluid restrictor at theoutlet 8, which again causes the upward movement of the piston 24.

Likewise, fluids with sufficiently low viscosity such as water or gaswould not create sufficient pressure in chamber B to maintain the piston24 in a raised position, thereby causing a closure of the outlet 8. Anupper chamber 28 shown above the piston 24 is set in fluid communicationwith the outside of housing 3 i via an upper conduit 29, thus ensuring aconstant downward force of the actuator 4 i which corresponds to theprevailing exterior pressure (p₁).

FIG. 10 illustrates an alternative embodiment as disclosed above forFIG. 9 in which the porous material 1 ensuring laminar flow during useis instead arranged within the upper conduit 29, and a channel/nozzle 30is introduced that extends from the upper chamber 28 and into the outputarea 31 located downstream of the output 8. In this embodiment thesecondary conduit 19 j corresponds to the flow through the upper conduit29 and the channel/nozzle 30.

In use, a minor portion f of the fluid flow F enters the housing 3 jthrough the upper conduit 29 and the porous material 1, and furtherthrough the channel/nozzle 30 into the output area 31. At the same timea major portion F₀ of the fluid flow F flows through inlet 7 into theflow path 18 j. The porous material 1 and the channel/nozzle 30 are thusacting as the first flow restrictor 1 and the second flow restrictor 2,respectively, while the upper chamber 28 has the same function aschamber B in FIG. 9. If fluids with sufficiently high viscosity such asoil are flowing into conduit 29, the moving member/actuator 4 j is in anopen position since the high flow resistance from the reactive material1 creates a correspondingly low pressure in the upper chamber 28 (B),i.e. not sufficient to force the valve member 24 downward and thuscausing a closure of the outlet 8. On the other hand, if fluids withsufficiently low viscosity such as water or gas are flowing into theupper conduit 29, the low resistance from the porous material 1 causes acorrespondingly high pressure in the upper chamber 28 sufficient toprovide a pressure on the surface 5 j of the actuator 4 j being highenough in order to move the valve member 24 downward, thus closing theoutput 8.

FIG. 11 illustrates a further embodiment of the inventive flow controldevice, wherein the housing 3 k is constructed having an actuatorchamber 28 in its interiors. The device further comprises an inlet 7, asecondary inlet assembly 7′, an outlet 8 and a moving member/actuator 4k situated inside the actuator chamber 28, which actuator 4 k comprisesa piston 24 and spring 25 connected to the interior walls of the chamber28 by appropriate seals 16 k. The secondary inlet assembly 7′ isarranged upstream of the actuator 4 k forming in its interior a pressurechamber B having an opposite situated opening with a porous material 1.Furthermore, one or more channels/nozzles 30 are introduced extendingfrom the chamber B and completely through or around the actuator 4 k,thereby forming a second conduit 19 k defined by the secondary inletassembly 7′ and the one ore more channels/nozzle 30. Except for theintroduction of a porous material 1 at the secondary inlet assembly 7′,thus forming a mainly laminar flow into the pressure chamber B duringuse, and the introduction of channel(s)/nozzle(s) 30 through or aroundthe moving member/actuator 4 k, thus forming a mainly turbulent flowduring use, the structural design of the device is similar or identicalto the device disclosed in the publication US 2011/0198097 A1, whichhereby is included by reference.

In use, a fluid flow F enters a primary conduit 18 k through a primaryinlet 7. This fluid flow is then divided into a major portion F₀ of theflow F going around the chamber 28 and a minor portion f of the flow Fentering the pressure chamber B through porous material 1.

The minor portion f further into the actuator chamber 28, subsequentlythrough the channel(s)/nozzle(s) 30 and finally through the outlet 8together with the major portion F₀ of the flow F. The porous material 1and the channel(s)/nozzle(s) 30 are thus acting as the first flowrestrictor 1 and the second flow restrictor 2, respectively, and thepressure chamber B has the same function as chamber B in FIG. 10. Iffluids with sufficiently high viscosity such as oil are flowing into thepressure chamber B the moving member/actuator 4 k is in an open positionsince the high flow resistance induced by the porous material 1 causes acorrespondingly low pressure (p₂) in the pressure chamber B, i.e. notsufficient to force the piston 24 sideways, thus resulting in a closureof the outlet 8. On the other hand, if fluids with sufficiently lowviscosity such as water or gas are flowing into the pressure chamber Bthe lower resistance set up by the porous material 1 compared with thehigh viscosity fluid, and the correspondingly high resistance at thechannel/nozzle 30, causes a correspondingly high pressure in thepressure chamber B sufficient to move the piston 24 sideways, thusclosing the output 8.

FIG. 12 illustrates a further embodiment of the inventive flow controldevice. The housing 3 l comprises an upper part 11 l (left slantedlines) and a lower part 12 l (right slanted lines), the upper and lowerparts 11 l,12 l being joined together by a threaded connection 20.Various seals 16 a-c (e.g. o-rings) are illustrated in the figure toprevent fluid from leaking between the upper and lower parts 11 l,12 l.The housing 3 l has an inlet 7 and radially arranged outlets 8, therebysetting up a primary conduit 18 l for the fluid F. A member 4 l isarranged for movement (in the figure: up and down) inside the housing 3l, guided by a supporting structure 17. Furthermore, a secondary conduit19 l is arranged from the inlet 7 and extending along the inside wallsof the housing 3 l, via a chamber B extending beneath the member 4 l,that is at the side of the member 4 l opposite to the primary conduit 18l, and ends in fluid communication with the outside of the housing 3 lat the lower part 12 l. A first fluid restrictor 1 in the form of aporous member are situated near the inlet 7 in the upper part 11 l and asecond fluid restrictor 2 in the form of an orifice are situated in thelower part 12 l, the second fluid restrictor 2 being in fluidcommunication with the outside of the housing 3 l. Chamber B thusextends from downstream the first fluid restrictor 1 to upstream thesecond fluid restrictor 2.

In use, a fluid flow F (e.g. oil from a subterranean reservoir) entersthe housing 3 l through the inlet 7. Inside the housing 3 l, the majorportion F₀ of the fluid F follows the primary conduit 18 l before itexits the housing 3 l through the outlet(s) 8. A portion f of the fluidF flows through the porous member 1 into the secondary conduit 19 l, viathe chamber B beneath the member 4 l and finally exits the chamber Bthrough the orifice 2. In this embodiment of the control device, thedifference in values for p₂ (ΔP₂, see FIG. 1b ), as the fluid viscosityμ changes, is serving to exert a pressure against a surface 5 l on themovable member and to close the movable member 4 l against the inlet 7.The sealing surfaces 14, 15 are brought together and thus substantiallyprevent fluid F from entering the conduit 18 l.

Note that for all the above embodiments the invention is not limited tospecific material such as porous member for the first or second fluidrestrictors or a specific geometry such as an orifice for the otherfluid restrictor. In fact, any choice of material and/or geometry ispossible as long as one of the restrictors creates a mainly laminar flowand the other restrictor creates a mainly turbulent flow during use.Also, even if directional words such as up, down, below, above,sideways, etc are used with reference to the drawings, in should beunderstood that these words are used only for clarity and should not beinterpreted as limiting the directional position of the inventivecontrol device.

All of the embodiments of the inventive flow control device describedabove are autonomous in that they move (to close or open a fluid inlet)based on a changing property (e.g. viscosity μ) of the fluid. The porousmember 1, the orifice 2 and the internal dimensions of the housing 3 a-kmay be designed to suit various applications.

As a first example, reference is made to FIGS. 13a and 13b , showingforces (E) acting on the movable piston 4 b-1 in an autonomous flowcontrol device configured for stopping water from entering the desiredoil flow phase as a function of pressure drop (p₁−p₃) across the flowcontrol device. E_(O) denotes the force that opens the control device,while E_(C) denotes the force that closes the device. It is seen that,while the fluid control device is open when subjected to oil(E_(o)>E_(c)) (FIG. 13a ), it closes almost instantaneously whensubjected to water (E_(o)<E_(c)) (FIG. 13b ).

For a second example, reference is made to FIG. 14, showing forces (E)acting on the movable piston in an autonomous flow control deviceconfigured for stopping any fluid inflow when the pressure differentialexceeds a given limit. E_(O) denotes the force that opens the controldevice, while E_(C) denotes the force that closes the device. It is seenthat the fluid control device closes at pressure drop (p₁−p₃) ofapproximately 8 bar.

These examples are intended to illustrate the function of the inventiveinflow control device. It should be understood that the fluid flowrestrictors 1,2 may be arranged and configured differently, for exampleessentially reversed in the flow path, if the device is intended to beused in a gas reservoir and it is desirable to prevent water fromentering the production.

It should be understood that the inventive flow control device may alsobe arranged and configured to control and prevent the inflow of otherfluids, such as CO₂ (which has been injected into the reservoir) andsteam (injected in connection with e.g. so-called Steam-Assisted GravityDrainage (SAGD) of heavy oil), and water in gas-producing wells.

Although the invention has been described with reference to the controlof well fluids (such as oil, gas, water) from a subterranean reservoir,the skilled person will understand that the invented device and methodis useful in any application where the objective is to control fluidflow based on the properties (e.g. viscosity, density) of the variousfluids in the flow in order to prevent unwanted fluids from entering afluid flow. Examples of such applications are injection wells,separation processes and steam traps.

What is claimed is:
 1. A downhole fluid flow control system comprising:a fluid control module having an upstream side and a downstream side,the fluid control module including a main fluid pathway in parallel witha secondary fluid pathway each extending between the upstream anddownstream sides; a valve element disposed within the fluid controlmodule, the valve element operable between an open position whereinfluid flow through the main fluid pathway is allowed and a closedposition wherein fluid flow through the main fluid pathway is prevented;a viscosity discriminator disposed within the fluid control module, theviscosity discriminator having a viscosity sensitive channel that formsat least a portion of the secondary fluid pathway; and a differentialpressure switch operable to shift the valve element between the open andclosed positions, the differential pressure switch including a firstpressure signal from the upstream side, a second pressure signal fromthe downstream side and a third pressure signal from the secondary fluidpathway, the first and second pressure signals biasing the valve elementtoward the open position, the third pressure signal biasing the valveelement toward the closed position; wherein, a magnitude of the thirdpressure signal is dependent upon the viscosity of a fluid flowingthrough the secondary fluid pathway; and wherein, the differentialpressure switch is operated responsive to changes in the viscosity ofthe fluid, thereby controlling fluid flow through the main fluidpathway.
 2. The flow control system as recited in claim 1 wherein thevalve element has first, second and third areas and wherein the firstpressure signal acts on the first area, the second pressure signal actson the second area and the third pressure signal acts on the third areasuch that the differential pressure switch is operated responsive to adifference between the first pressure signal times the first area plusthe second pressure signal times the second area and the third pressuresignal times the third area.
 3. The flow control system as recited inclaim 1 wherein the viscosity discriminator further comprises aviscosity discriminator disk.
 4. The flow control system as recited inclaim 3 wherein the main fluid, pathway further comprises at least oneradial pathway through the viscosity discriminator disk.
 5. The flowcontrol system as recited in claim 3 wherein the viscosity sensitivechannel further comprises a tortuous path of the viscositydiscriminator.
 6. The flow control system as recited in claim 5 whereinthe tortuous path is formed on a surface of the viscosity discriminator.7. The flow control system as recited in claim 5, wherein the tortuouspath further comprises at least one circumferential path.
 8. The flowcontrol system as recited in claim 5, wherein the tortuous path furthercomprises at least one reversal of direction path.
 9. The flow controlsystem as recited in claim 1, wherein the third pressure signal is froma location downstream of the viscosity sensitive channel and wherein thethird pressure signal is a total pressure signal.
 10. The flow controlsystem as recited in claim 1, wherein the magnitude of the thirdpressure signal increases with decreasing viscosity of the fluid flowingthrough the secondary fluid pathway.
 11. The flow control system asrecited in claim 1, wherein the magnitude of the third pressure signalcreated by the flow of a desired fluid through the secondary fluid pathshifts the valve element to the open position and wherein the magnitudeof the third pressure signal created by the flow of a undesired fluidthrough the secondary fluid path shifts the valve element to the closedposition.
 12. A downhole fluid control method comprising: positioning afluid flow control system at a target location downhole, the fluid flowcontrol system including a fluid control module having an upstream sideand a downstream, a viscosity discriminator and a differential pressureswitch, the fluid control module including a main fluid pathway inparallel with a secondary fluid pathway each extending between theupstream and downstream sides, the viscosity discriminator having aviscosity sensitive channel that forms at least a portion of thesecondary fluid pathway; producing a desired fluid from the upstreamside to the downstream side through the fluid control module; operatingthe differential pressure switch to shift the valve element to the openposition responsive to producing the desired fluid by applying a firstpressure signal from the upstream side to a first area of the valveelement, a second pressure signal from the downstream side to a secondarea of the valve element and a third pressure signal from the secondaryfluid pathway to a third area of the valve element; producing anundesired fluid from the upstream side to the downstream side throughthe fluid control module; and operating the differential pressure switchto shift the valve element to the closed position responsive toproducing the undesired fluid by applying the first pressure signal tothe first area of the valve element, the second pressure signal to thesecond area of the valve element and the third pressure signal to thethird area of the valve element; wherein, a magnitude of the thirdpressure signal is dependent upon the viscosity of a fluid flowingthrough the secondary fluid pathway such that the viscosity of the fluidoperates the differential pressure switch thereby controlling fluid flowthrough the main fluid pathway.